1. Field of the Invention
The present invention relates to baluns and more particularly to Guanella style baluns.
2. Description of the Related Art
It has long been known that an enclosure of conductive material will block electrical fields. That is, electrical fields inside the enclosure do not leak out and electric fields outside the enclosure do not get inside of the enclosure. The enclosure does not need to be continuous (for example, it may be a mesh) but it must be at least somewhat continuous to substantially block passage of electrical fields in or out of the enclosure. For example, U.S. Pat. No. 3,237,130 (“130 Cohn”) discloses a directional coupler for microwave wavelength signals that includes conductive enclosures, specifically an intermediate conductor (see 130 Cohn at its FIG. 4A, reference numeral 58) and an outer conductor (see 130 Cohn at its FIG. 4A, reference numeral 52). The use of a conductive enclosure around transmission lines, as in the couplers of 130 Cohn, is sometimes referred to as a re-entrant enclosure, and the coupler of 130 Cohn is called a re-entrant coupler.
A balun is a known type of electrical component. A balun is a two port electronic device that transforms a signal between a signal suitable for a balanced transmission line and a signal suitable for an unbalanced transmission line. The two ports of the balun are referred to as the unbalanced port (referenced to ground) and the balanced port (two terminals, one referenced to the other). Applications for balun transformers include antenna feed circuitry, push-pull amplifiers, circuitry for splitting and combining signals and other circuitry where transformation from balanced to unbalanced signals is required.
One type of known balun is the “Guanella balun,” a well known circuit configuration. Guanella baluns are characterized by their impedance ratios. More specifically, an x:y Guanella balun has an unbalanced port impedance that is x/y times its balanced port impedance. Some popular types of Guanella baluns are 1:1 Guanella baluns, 1:4 Guanella baluns and 4:1 Guanella baluns. 1:1 Guanella balun circuits may include only a single set of coupled, but often Guanella baluns will have multiple sets of coupled lines.
Guanella baluns use magnetic material (usually ferrite) to increase the impedance seen by common mode currents. This increase in common mode impedance is paramount to the performance of the balun. The higher the common mode impedance, the better the balun performance. The performance of a Guanella balun can be understood by considering the transmission line model 100 as seen in FIG. 1A. The reactance of the windings will affect the common mode impedance and therefore set the low frequency performance. Ferrite material can be used to increase this reactance. Notice in model 100, if node 5 is connected to ground, the device can be considered a three port device with the balance port load (RL) now split into two separate loads each of which is referenced to ground (see, path from node 4 to node 5 and path from node 2 to node 5). The ports will all be matched when the two separate loads are each RL/2. The same is true of a load connected between the two terminals of the balanced port in the coupled transmission line model 150, shown in FIG. 1B. It is desirable to have the same amplitude at each of the two loads when a signal is applied to the unbalanced port. The difference between the amplitudes at the two loads is called amplitude balance. The signals should also be 180 degrees out of phase and any error from this phase difference is called phase balance.
More recently, the analysis has been extended to use coupled transmission lines (i.e., couplers) as shown in FIG. 1B. As shown in FIG. 1B, the coupled transmission lines are surrounded by magnetic material 151 which characterizes a Guanella type balun. When a coupled line structure is used to implement the Guanella 1:1 balun circuit, the coupler parameters will define the port impedances and the frequency of operation. The coupler parameters are adjusted by changing material properties and the circuit configuration. If the balun is implemented using coupled lines, increasing the coupler even mode impedance (Zeven) will increase the impedance seen by common mode currents. The higher Zeven, the better the balun will perform. The frequency band of operation will be centered at the frequency where the coupler electrical length is 90 degrees (or one quarter wavelength). The bandwidth will increase with increases in Zeven. The balun will not work when the coupled line section is 0 degrees (DC) or at odd multiples of 180 degrees.
FIG. 2A shows coupled line structure schematic 175 including: dielectric material 179; ground planes 177; and plane of symmetry 181. FIG. 2B shows odd mode field pattern schematic 200 including perfect electric wall 202. FIG. 2C shows even mode electric field pattern schematic 225 including 225 including perfect magnetic wall 227. Guanella baluns have been constructed using stripline broadside couplers. This type of coupled line structure is often analyzed using the well established even/odd mode analysis illustrated in FIGS. 2A, 2B and 2C. With broadside coupled lines, the odd mode fields are mostly, but not all, contained between the coupled lines. For example, as shown in odd mode schematic 200, numerous field lines 204a run between the coupled lines, while relatively few field lines 204b do not run between the coupled lines. As shown in schematic 225, the even mode fields are primarily concentrated between the traces and the ground planes. It is desirable for balun performance to increase the impedance seen by the even mode fields while keeping the odd mode impedance constant at Zodd. A few equations relating the coupler impedances to the 1:1 balun port impedances are:Zunbal=Zbal  Eq. (1):Zodd=Zbal/2  Eq. (2):Zeven>20×Zodd; for reasonable operation—the higher the better.  Eq. (3):
In Equations (1) to (3): (i) Zunbal=Unbalanced (single ended) port impedance of the balun; (ii) Zbal=Balanced port impedance of the balun; (iii) Zeven=even mode impedance of the coupler used to construct the balun; and (iv) Zodd=odd mode impedance of the coupler used to construct the balun.
The odd mode impedance can be adjusted by changing the line width, spacing between the lines and material properties. The even mode impedance can be set as high as possible by making the coupled lines narrow and increasing the distance between the coupled lines and the ground planes. Any of these adjustments should preferably be made while simultaneously maintaining the desired odd mode impedance and increasing the even mode impedance. From Eq. (1), Zeven should be 20 times greater than Zodd. When this is the case, the ideal response for a [5.7:1] frequency bandwidth would be characterized as follows: (i) amplitude balance ≈1.0 dB (0.9 dB at Fc); (ii) phase balance ≈±10 degrees (170 degrees at Flow, 180 degrees at Fc, 190 degrees at Fhigh); and (iii) return loss ≈26 dB.
This would not be acceptable for many applications and the Zeven required for these conditions is very difficult to achieve (while maintaining the required Zodd) using conventional, dielectric only coupled line structures. Spiral couplers have been investigated for the purpose of increasing even mode impedance but in general require very narrow coupled transmission lines, which severely limits the power handling capability of the balun.
FIG. 3A shows coupled line structure schematic 250 including: ground planes 252; dielectric material 254; ferrite material 256; and plane of symmetry 258. FIG. 3B shows odd mode field pattern schematic 275 including perfect electrical wall 277. FIG. 3C shows even mode field pattern schematic including perfect magnetic wall 302. FIGS. 3A, 3B and 3C illustrate the addition of magnetic material (usually ferrite) in the construction of a balun using broadside coupled lines. The coupled lines are still printed on dielectric material, but the dielectric material is enclosed in the ferrite material. The field patterns change based on the magnetic material properties. The magnetic materials are characterized by their permeability (μ) and permittivity (∈). Both of these parameters are complex values with real parts μ′ and ∈′ and imaginary parts μ″ and ∈″. The imaginary parts represent the loss associated with the material. In general, Zeven will be increased due to the relatively large permeability (μ) of the magnetic material. Most dielectric materials have μ=1 but some known ferrite materials have μ>1. When η>1 the inductance of the transmission line will increase and the impedance will also increase. This is due to the impedance Equation (4) and a related Equation (5). Eq. (4): Z=squareroot (L/C). Eq. (5): Z α squareroot (μ/∈)
The μ and ∈ will also affect the electrical length of the coupled lines. Increasing μ or ∈ will increase the propagation constant as shown by Equations (6) and (7). Eq. (6): β=ω (squareroot (LC)). Eq. (7): ω α squareroot (μ∈).
One design for a Guanella balun is disclosed in U.S. Pat. No. 5,808,518 (“518 McKinzie”). 518 McKinzie a Guanella type balun (with its characteristic magnetic material) that uses strip transmission lines, as opposed to the other types of coupled lines. 518 McKinzie is directed to a 1:4 Guanella balun and therefore has two pairs of couple transmission lines. Each pair of strip transmission lines is located in its own respective magnetic material enclosure, specifically an enclosure in the form of a tube.
The following published documents may also include helpful background information: (i) US patent application 2008/0246679 (“679 Martek”); (ii) US patent application 2003/0003776 (“776 Lohr”); (iii) US patent application 2009/0045886 (“Gruchalla”); and (iv) “The Re-entrant Cross Section and Wide-Band 3-dB Hybrid Couplers,” by Seymour B. Cohn, IEEE Transactions on Microwave Theory and Techniques, July 1963.
Description Of the Related Art Section Disclaimer: To the extent that specific publications are discussed above in this Description of the Related Art Section, these discussions should not be taken as an admission that the discussed publications (for example, published patents) are prior art for patent law purposes. For example, some or all of the discussed publications may not be sufficiently early in time, may not reflect subject matter developed early enough in time and/or may not be sufficiently enabling so as to amount to prior art for patent law purposes. To the extent that specific publications are discussed above in this Description of the Related Art Section, they are all hereby incorporated by reference into this document in their respective entirety(ies).